Magnetic means for focusing and densifying the electron beam in traveling wave tubes



June 4, 1963 w. vErrH ETAL 3,092,745

MAGNETIC MEANS FOR FOCUSING AND DENSIF'YING THE ELECTRON BEAM IN TRAVELING WAVE TUBES Filed June 24, 1959 2 Sheets-Sheet 1 N s s N N -uou HOMOGENOUS RANGE PRE0ETERM|NE0 RANGE OF THE MAGNETIC FIELD OF THE MAGNETIC FIELD Fig.2

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MAGNETIC MEANS FOR FOCUSING AND DENSIFYING THE ELECTRON BEAM IN TRAVELING wAvE TUBES Filed June 24, 1959 2 Sheets-Sheet 2 Fig.3

NON HOMOGENOUS RANGE IELD OF THE MAGNETIC FIELD MAGNETIC F wer fani Q/srizer 6, Paw/ fife/6r.

United States Patent MAGNETIC MEANS FOR FOCUSHNG AND DENSI- FYING THE ELECTRGN BEAM 1N TRAVELWG WAVE TUBES Werner Veith and Paul Meyer-er, Munich, Germany, assignors to Siemens & Halske Aktiengesellschaft, Berlin and Munich, Germany, a corporation of Germany Filed June 2.4, 1959, Ser. No. 822,592 Claims priority, application Germany June 25, 1958 11 Claims. (Cl. 313-84) This invention is concerned with an electron beamproducing system for tubes having a magnet system for guiding the focused and densified electron beam, such system producing a magnetic field which is in discharge direction over the largest discharge range homogeneous or alternating, the system comprising a cathode lying in the non-homogeneous range of the magnet field which in electron radiation direction precedes the homogeneous or alternating range of the magnet field, and having a ring-shaped Wehnelt cylinder and one or more anodes.

Electron beam-producing systems have become known, comprising electrostatic means disposed directly adjacent the cathode for the densifying of the electron flow emanating from the cathode. It is in these known beamproducing systems important that the densifying of the electron stream is solely effected by the electrostatic means. In the known electron guns, the magnet field will begin to influence the homogeneous or alternating magnet field only after the stream of electrons converging from the cathode is densified to such extent that the further guiding of the bundles or focused electrons serves solely the purpose of maintaining the same cross section of the electron stream. The theoretically ideal condition is in these known electron beam-producing devices obtained when the converging and densifying of the electron flow is effected solely by the electrostatic system and when the field strength in the direction of the electron beam, required for the further guidance of the bundled electron beam, becomes suddenly operative at the place at which the desired densifying is reached. This condition cannot be realized in practical operation. The known arrangement therefore provides a practically feasible dimensioning rule according to which the electron beam producing system is to be disposed in the non-homogeneous area of the magnet field, which in the electron beam direction precedes the homogeneous range, in such a manner, or together with auxiliary shielding cylinders or opposing field coils, that the magnetic field lines mainly follow the converging path of the electrons.

It was found in the use of the known electron beamproducing systems for tubes with a magnet system for the guidance of the focused md densified electron beam, especially in the use of traveling field tubes in which the electron beam must be guided very closely to delay lines, that operatively effective adjustment requires alignment of three system axes. The first axis is that of the electron beam-producing system, the second that of the delay line, and the third that of the magnet system.

The utilization of ever higher frequencies, requiring small electron beam cross sections, high current density and smallest dimensioning of the delay lines, results in the drawback that the above mentioned adjustment of the three system axes cannot be readily effected.

In the exchanging of traveling field tubes, in which the magnet system is not exchanged, it was moreover found that a subsequent adjustment of the above noted three system axes is not always simple and requires a great deal of time.

It is accordingly, the object of the invention to reduce the number of axes that have to be mutually adjusted.

The essential feature of the electron beam producing system according to the invention, for tubes with a magnet system for the guidance of the bundled and densified electron beam, consists in forming the electron emitting end of the cathode which extends approximately perpendicular to the radiation axis, so that the electron beam rim, after leaving the emission layer, diverges or exhibits a slighter convergence than the cone between cathode rim and radiation rim of the densified electron beam.

In the expression for the magnetic focusing force which is equal to the Lorenz force reduced by centrifugal force The action of the magnet field B enters by the square and the radiation rim radius 17 of the beam enters linearly. K represents the shielding constant. The focusing action of the magnet field will be greater with greater diameter of the beam. It follows that upon divergent exit of the electrons from the emitting surface of the cathode, the focusing action is in the presence of a magnet field greater than in the case of convergent exit. Since the cathode lies in the non-homogeneous range of the magnet field and since the electrons leave the cathode divergent or not as strongly convergent as in the known systems, the focusing action or the force produced by the magnet field on the individual electrons is as compared with previously known systems very much greater. This operation, the object of which is that the electrons leaving the cathode immediately intersect as many magnetic field lines as possible, forces the electron beam, by the action of the magnetic field, into the magnetic system axis. The position of the system axis of the electron gun accordingly does not practically affect the further course of the electron beam.

The requirements placed on the further beam course are very high. The electron density is to be constant in the beam cross section. The beam rim shall describe the genetrix of a cylinder without bulges. This cylindrical beam shall maintain its shape and shall not spread even when affected by high frequency fields which may assume high intensity at the beam end. This requirement is largely fulfilled when the shielding constant K is as great as possible. The shielding constant is defined by the equation wherein B is the magnetic induction, b the beam radius, and the index k indicates that the corresponding values refer to the cathode. It will be already apparent from the equation that a finite shielding constant Knamely a finite magnetic field on the cathodeis in contradiction to the previously described electron beam systems in which the cathode surface is to be essentially free of magnetic fields.

The invention therefore results, in addition to reducing the system axes that have to be mutually adjusted, in the further considerable advantage of producing in the presence of a magnetic field on the cathode surface improved beam qualities resulting from the higher shielding constant.

The electron emitting end of the cathode may be plane or convex for obtaining a sufficiently high divergence of the electron beam surface and electron beam rim, re spectively. Moreover, the aperture diameter of the accelera-tion anode following the cathode in the direction of discharge, may for the same purpose be greater than the diameter of the emitting cathode surface. For supporting the action of the magnetic field on the electron beam and shortening of the range between the end of the cathode and the place at which the magnetic field strength reaches its highest value on the system axis, there may be provided two further electrodes forming an electrostatic lens following the acceleration anode in the electron beam direction.

It is furthermore advantageous to provide, within the range between the emission surface of the cathode and the place at which the magnetic field strength reaches its highest value on the system axis, approximately coaxially to the electron beam, a magnetically soft cylindrically shaped ring for increasing the action of the magnetic field directly ahead of the end of the cathode.

For reasons of better adjustment of the action of the non-homogeneous magnetic field on the electron beam and for adjustment with respect to the axis of the delay line, the magnetically soft ring may form part of the vessel Wall. To facilitate the regulation of the operative intensity of the magnetic field ahead of the cathode, a second cylindrically shaped and likewise magnetically soft ring may be provided upon the cylindrical ring in the vessel wall, and such second ring may be disposed thereon so as to be movable relative thereto in the direc tion of the electron beam.

The various objects and features of the invention will appear from the description of two embodiments thereof H which will be rendered below with reference to the accompanying drawings. In the drawings,

FIGS. 1 and 2 show one embodiment and the course of the corresponding magnetic induction in the electron beam axis;

FIGS. 3 and 4 show another embodiment and the course of the corresponding magnetic induction in the electron beam axis; and

FIG. 5 is a figure similar to FIG. 4 illustrating the course of the corresponding magnetic induction in the electron beam axis, employing a periodically effective magnetic field.

Parts which are not absolutely required for the understanding of the invention, such for example, as the magnet system, the coupling and decoupling devices for the electromagnetic wave, the target electrode, etc., have been omitted from the drawings. Like parts are indicated by like reference numerals.

FIG. 1 shows the course of the magnetic induction B along the electron beam axis z. In the cathode plane K, the magnetic induction B of the non-homogeneous range of the magnetic field is as compared with the highest magnetic induction B much lower. In the range between the cathode plane K and the place V, at which the magnetic field strength has reached its highest value on the system axis, there appear the phenomena which are decisive for the invention so far as the action of the magnetic field on the electron motion is concerned.

As will be seen from FIG. 2, showing the electron stream 3 as it leaves the emission surface of the cathode 8, the beam 3 spreads very considerably after leaving the emission surface 1, thus becoming divergent. The divergence of the electron beam in the vicinity of the emitting surface '1 of the cathode 8 is in the main effected by the plane emission surface 1, by the exit forces of the electrons leaving the cathode and by the dimensioning of the acceleration anode 2. The aperture diameter of the acceleration anode exceeds that of the emission surface 1. The anode 2 accordingly forms by the acceleration voltage connected thereto and by the larger aperture opening, a divergence lens. The further course of the beam 3 in the z-direction is determined by the increase of the magnetic induction B and by the relatively large beam rim radius b. The magnetic focusing force thereby becomes so great that it produces in the further course a convergence of the beam 3 in the z-direction.

A Wehnelt cylinder 4 is provided in similar manner as in known electron beam-producing systems (Pierce- Kanone), having suitably shaped surfaces, such cylinder determining in known manner the field conditions for the beam rim.

The lens composed of the electrodes 2, 6 and 7 serves for correctly securing the injection of the electron beam into the path 5, represented by a helix, in which the magnetic field is constant. The voltages at the electrodes are, for example, such that electrodes 2 and 7 are on the full anode voltage while the intermediate electrode 6 is on a variable lower voltage. The lens (2, 6, 7) supports the focusing action of the magnetic field and permits adjustment to any desired injection angle.

The electron gun and electrodes are illustrated as disposed in a discharge vessel or envelope 10, made of glass :or ceramic material and the helix 5, determining the electron course, is contained in the discharge vessel 9. Between the parts 9 and 10 is disposed a cylindrical ring 11 made of magnetically soft material, for example, Kovar.

Disposed about the vessel 9 is a periodic permanent magnet system 13, of known type, operative to produce an alternating magnetic field region, in which, as illustrated in FIG. 5, the course of the magnetic induction B in the z-direction, will be mainly sinusoidal, the z-axis forming the Zero axis for the sine wave. a

FIG. 3 shows the envelope or vessel part containing the electron gun and its most important parts. The cathode 8 having the emission surface 1 and the Wehnelt cylinder 4 are disposed within the discharge vessel or envelope 1! made of glass or ceramic material, and the helix 5 determining the electron course is contained in the discharge vessel 9. Between the part 10 and the part 9 of the discharge vessel is disposed a cylindrical ring 11, made of magnetically soft material, for example, Kova-r, such material being also very well suitable for sealing purposes. The provision of the ring 11 secures the course of the magnetic induction B as shown in FIG. 4. A further ring 12, which may be made of soft iron, is provided upon the cylindrical member 11 and is movable thereon in the z-direction. The adjustment of the ring 12 on the ring 11 effects shifting of the maximum magnetic induction which is primarily effective ahead of the cathode, in its position M between the points K and V. The height of such maximum is also varied, as shown by the dash line curves in FIG. 4. The action of the magnetic field ahead of the cathode 8 can be regulated by the shifting of the ring 12.

The ring 111, in addition to the influencing of the magnetic field may take over the function of the acceleration anode :of the electron beam-producing system or may carry the acceleration anode and also serve as terminal member.

Disposed about the vessel 9 is a magnet coil 14, operative to produce a homogeneous magnetic field, thereby providing for a course of magnetic induction B as illus trated in FIG. 4. A strong magnetic field is in such case obtained very closely adjacent the cathode 8. It has been found that the length of the field rise between the points K and V is considerably reduced by the use of a magnetic field corresponding to the course of the magnetic induction B in z-direction whereby, as illustrated in FIG. 4, the magnetic field, extending from the cathode in the discharge direction, between the latter and its homogeneous range, passes through a maximum and a minimum.

The invention is not inherently limited to an arrangement having a plane cathode but may, for example, very well be used with a cathode with diverging or slightly converging beams. It is only essential that the relatively weak magnetic field in the non-homogeneous range embraces the beam in the vicinity of the cathode and that it forces the beam in the direction of the axis of the magnetic field.

Beam-producing systems according to the invention are suitable for all kinds of microwave tubes. The described arrangement is, especially in systems with very strongly densified beam, for example, with beam diameters less than one millimeter, as used in tubes for very short waves (millimeter waves), not only very advantageous but represents in very many instances the only possible solution for the problems involved. The reason therefor is seen in the fact that the rise of the field, in connection with the very high magnetic field strengths exceeding 1000 gauss, as used with millimeter wave tubes, does not take place even approximately abruptly, requiring in most instances a longer path as required in an electron beam system with the known strongly convergent electron stream.

Changes may be made Within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

We claim:

1. An electron beamproducing system for discharge tubes with a magnetic system for the guidance of a focused and densified electron beam, said magnetic system producing a predetermined magnetic field extending in the discharge direction over a major part of the discharge range, the cathode of said system being disposed in the nonhomogeneous range of the magnetic field which precedes the predetermined range thereof as seen in the electron beam direction and emitting an electron beam with nonconvrging borders, a ring-shaped Wehnelt cylinder and at least one anode operatively spaced from said cathode, the magnetic field extending between the cathode and the predetermined range of said field passing through a maximum and a minimum and a magnetically soft cylindrically shaped ring disposed approximately coaxially with the electron beam Within the region between the emitting cathode surface and the point of highest value of the magnetic field strength upon the system axis to increase the action of the magnetic field directly ahead of the end of the cathode.

2. An electron beam-producing system as defined in claim 1, wherein said magnetic system produces a homogeneous magnetic field.

3. An electron beam-producing system as defined in claim 1, wherein said magnetic system produces an alternating magnetic field.

4. An electron beam-producing system according to claim 1, wherein the magnetically soft ring is disposed so as to connect an enlarged portion of the vacuum tube envelope, which contains the Wehenelt cylinder and the anodes, with a narrower part containing a delay line.

5. An electron beam-producing system as defined in claim 1, wherein said magnetically soft cylindrically shaped ring forms a part of the vacuum envelope of the tube.

6. An electron beam-producing system as defined in claim 1, comprising an acceleration electrode disposed as seen in discharge direction following the cathode and having an aperture formed therein with a diameter exceeding that of the emitting surface of the cathode.

7. An electron beam-producing system as defined in claim 6, comprising two further electrodes disposed successively following said acceleration electrode and forming an electrostatic lens.

8. An electron beam-producing system as defined in claim 1, comprising a further cylindrically shaped magnetically soft ring carried by said first named ring and axially movable thereon.

9. In an electron tube having for the guidance of a focused and densified electron beam a magnet system which produces a predetermined magnetic field extending in the discharge direction over a major part of the discharge range, an electron beam-producing system comprising a cathode disposed within the range of the magnetic field which precedes said predetermined magnetic field, said preceding field passing through a maximum and a minimum, the electron emitting face of said cathode extending approximately perpendicularly to the axis of the beam to be produced, a Wehnelt cylinder disposed adjacent the electron emitting face of said cathode, an apertured acceleration electrode disposed as seen in discharge direction in spaced relation to said electron emitting face of said cathode and said Wehnelt cylinder, the inner diameter of the aperture of said acceleration electrode exceeding the outer diameter of the electron emitting face of said cathode, two further mutually spaced apart electrodes disposed as seen in discharge direction in spaced relation to said acceleration electrode, said further electrodes forming an electrostatic lens, whereby the electrons emanating from the emitting face of said cathode are caused to form a beam which diverges within an initial portion of its path in the vicinity of said acceleration electrode and thereupon converges cone-like to the end of its path so as to effect the densifying and focusing thereof, and a magnetically soft cylindrically shaped member disposed substantially coaxially with the path of the electron beam Within a region extending between the emitting face of said cathode and a point of highest value of the magnetic field strength upon the system axis to increase the action of the magnetic field directly ahead -:of the end of the cathode.

10. An electron tube as defined in claim 9, comprising a further magnetically soft cylindrical member carried by said first named member and axially movable thereon.

11. An electron tube as defined in claim 9, wherein said magnetically soft cylindrically shaped member forms a part of the envelope of said tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,431,077 Poch Nov. 18, 1947 2,781,472 Clavier Feb. 12, 1957 2,817,033 Brewer Dec. 17, 1957 2,828,434 Klein Mar. 25, 1958 2,843,790 Cutler July 15, 1958 2,844,750 Veith July 22, 1958 2,871,392 Rogers et al Jan. 27, 1959 2,876,378 McDowell Mar. 3, 1959 2,884,556 Iversen Apr. 28, 1959 2,905,847 Klein Sept. 22, 1959 

1. AN ELECTRON BEAM-PRODUCING SYSTEM FOR DISCHARGE TUBES WITH A MAGNETIC SYSTEM FOR THE GUIDANCE OF A FOCUSED AND DENSIFIED ELECTRON BEAM, SAID MAGNETIC SYSTEM PRODUCING A PREDETERMINED MAGNETIC FIELD EXTENDING IN THE DISCHARGE DIRECTION OVER A MAJOR PART OF THE DISCHARGE RANGE, THE CATHODE OF SAID SYSTEM BEING DISPOSED IN THE NONHOMOGENEOUS RANGE OF THE MAGNETIC FIELD WHICH PRECEDES THE PREDETERMINED RANGE THEREOF AS SEEN IN THE ELECTRON BEAM DIRECTION AND EMITTING AN ELECTRON BEAM WITH NONCONVERGING BORDERS, A RING-SHAPED WEHNELT CYLINDER AND AT LEAST ONE ANODE OPERATIVELY SPACED FROM SAID CATHODE, THE MAGNETIC FIELD EXTENDING BETWEEN THE CATHODE AND THE PREDETERMINED RANGE OF SAID FIELD PASSING THROUGH A MAXIMUM AND A MINIMUM AND A MAGNETICALLY SOFT CYLINDRICALLY SHAPED RING DISPOSED APPROXIMATELY COAXIALLY WITH THE ELECTRON BEAM WITHIN THE REGION BETWEEN THE EMITTING CATHODE SURFACE AND THE POINT OF HIGHEST VALUE OF THE MAGNETIC FIELD STRENGTH UPON THE SYSTEM AXIS TO INCREASE THE ACTION OF THE MAGNETIC FIELD DIRECTLY AHEAD OF THE END OF THE CATHODE. 